What Are Those Mysterious Radio Signals Coming From the Centre of the Milky Way?

Strange signals coming from an unidentified outer space object are baffling scientists internationally. The radio signals, which appear to issue from the centre of our galaxy, arrive sporadically and travel in a pattern that defies current understanding.

In a study released today, The Astrophysical Journal says that the signals first appeared in January 2020. After using a broad array of techniques to detect and interpret them, scientists on the project still don’t know what their source is.

“The strangest property of this new signal is that it has a very high polarization,” lead study author Ziteng Wang of the University of Sydney said in a press release. “This means its light oscillates in only one direction, but that direction rotates with time.

“The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off, apparently at random,” Wang added. “We’ve never seen anything like it.”

First Contact

The team first detected the radio signal with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in the Australian outback. It showed up regularly for the next couple of weeks, then disappeared without a trace.

A few months later, it suddenly came back.

“Sometimes it seems to stay on, detectable for days or weeks at a time, and then other times it can come on and off in a single day, which is extremely fast for an astronomical object,” Tara Murphy, University of Sydney Professor and study co-author said.

Murphy went on to say that the radio signal’s behavior “rules out almost all astronomical objects we know of.”

So…what is it?

With all questions and no answers, the team got to work.

In the months following the initial discovery, Wang led the far-flung group of scientists from Australia’s national science agency CSIRO, Germany, the United States, Canada, South Africa, Spain, and France in the attempt to identify the signal’s source.

They started with three options on the table. The first possibility was that it was a pulsar, the rapidly spinning heart of a dead star. Pulsars commonly send out regular bursts of high energy, so the team trained the Parkes Radio Telescope (famous for detecting pulsars) at it.

But they didn’t find anything. So the team relocated to South Africa to search for the object with the MeerKAT Telescope, which can detect pulses and capture images of the radio signals.

After three months of searching, the signal appeared. However, the team found that it didn’t display the properties of a pulsar or even a magnetar (another variety of dead, spinning stars).

unidentified radio signal from center of galaxy

Inset shows the radio signal switched off, then on, as demonstrated by the MeerKAT telescope. Photo: Tara Murphy


A solar flare?

The next possibility was that the object was a massive solar flare. The signal was so strong that the source should be visible. But it was completely invisible, so they ruled that out.

That left one possibility, anywhere within the realm of current knowledge. It could belong to a category of rare objects called galactic centre radio transients. Scientists have observed very few of these radio signals and have noted that they all behave somewhat differently.

It was an exciting proposition, but it came with a catch — nobody knows what galactic centre radio transients are.

“It could be that we’ve discovered one of these, so in a way that’s exciting, because there are very few of them known, but also frustrating because we don’t actually know what [they] actually are,” Professor Murphy said.

Cosmic burper? And if so, what’s a cosmic burper?

Galactic centre radio transients are also called “cosmic burpers”. Although we don’t know what they are, they share the behavior of this mystery object — they emit signals sporadically, often stopping entirely for weeks or months at a time.

All else about cosmic burpers is unknown. It’s a loose category, only understood in a rudimentary way. But with equipment like the ASKAP and MeerKAT telescopes, teams like Wang and Murphy’s may be well on their way to discovering more.

“[B]y building up a statistical number of them, we’ll be able to work out what they are,” Murphy said. “That’s how things often happen in astronomy; you find one rare thing, then you find more like it, and eventually you can actually understand what’s going on.”

For now, the mystery remains. Are cosmic burpers simply rogue chunks of inert material that we haven’t categorized yet? Some offshoot or cousin of pulsars? Or something else?